In the current standard for the time division duplex (TDD) communication method for mobile communication of the third generation (refers to the technical standards related to 3GPP), only the method for operation with single carrier is defined. The base station should be designed to be able to operate with multiple carriers in order to satisfy the communication need under the environment of applications of high density, however. In the “multi-carrier time division duplex mobile communication system” designed based on such a thought, the radio base station operates with multiple carriers, and multiple carriers, such as with three frequencies of f0, f1 and f2, may be used within one cell to increase the capacity of the cell, or more specifically, to increase the capacity of the system while lowering the cost of the system, and simplify the design of the system so that the system will be controlled and managed more easily, and the efficiency of the system will be increased.
However, only one set (or group) of radio transmitter/receiver is commonly used in one base station, under which condition all the carrier frequencies should be transmitted or received simultaneously when the base station operates with multiple carriers, otherwise, severe interference occurs inside the base station, which may cause transmitting or receiving failures in the system.
And when the cellular mobile communication system is constituted with conventional TDD system, when the same carrier frequency is used in adjacent cells, such adjacent cells must operate in sync, i.e., transmit the downlink signal simultaneously and receive the uplink signal simultaneously, according to the basic requirements of the CDMA mobile communication system, when the system is being scheduled by the operator, otherwise, interference occur between the base stations within the communication system.
The possibility for TDD system to support asymmetric services where different uplink and downlink ratios are used in adjacent cells is greatly constrained by the existence of above problems, or in other words, some system capacity must be sacrificed if asymmetric services where different uplink and downlink ratios are used in adjacent cells are supported.
FIG. 1 is a schematic view showing the operation state of multi-carrier base stations in a typical TDD cell mobile communication system wherein the 3GPP standard is performed. Three cells 101, 102, and 103 are shown in FIG. 1, wherein a single carrier frequency or multiple carrier frequencies, such as three carrier frequencies f0, f1 and f2, which are the same among the cells are used by the base station 111, 112, and 113 of respective cells, and a plurality of terminals in operation exist in respective cells. In the present normal operation state, the switching between transmission and receiving must be performed at the same time for all three carrier frequencies of respective cells, i.e., the downlink transmission or the uplink receiving must be performed at the same time for all the carrier frequencies of the base stations in respective cells, whereby the interference between respective frequencies during the overlapping time slot between the uplink and downlink can be avoided. The adjacent cells should also be coordinated to adapt the same switching time (at the same switching point) between uplink and downlink, and if different switching times are used in respective cells, for example, different switching times between uplink and downlink t1, t2, and t3 are adapted for three carriers f0, f1 and f2 in the base station 111, 112, and 113 of respective cells, interference will be caused during the overlapping time slot between the uplink and downlink.
FIG. 2 schematically shows the conventional structure design of a base station in the TDD system which includes a radio transmitter/receiver and an antenna system. Normally, the antenna system comprises n antennae 201, 202, . . . , 20n, and n feeding cables 211, 212, . . . , 21n corresponding to the n antennae 201, 202, . . . , 20n, respectively. The n antennae are connected to n radio receivers 231, 232, . . . , 23n and n radio transmitters 241, 242, . . . , 24n on desks in the room via the n feeding cables respectively, which are connected to the baseband signal processing unit 251. In TDD systems, the switching between transmitting and receiving is performed through elements 221, 222, . . . , 22n, such as radio switches or circulators, i.e., a set of antenna and feed line may corresponds to a set of radio receiver/transmitter.
The normal operation state in which the same switching time between uplink and downlink are used for respective carrier waves in respective cells and the abnormal operation state in which different switching times between uplink and downlink are used for respective carrier waves in respective cells will be explained with reference to FIG. 3. The downlink f0, f1, and f2 are represented by rectangles with horizontal line patterns, right slanting line patterns, left slanting line patterns respectively, and the uplink f0, f1, and f2 are represented by rectangles with small square patterns, blank patterns and grey patterns respectively. As for the example shown in FIG. 1, the same switching time t1 is used for respective carrier frequencies f0, f1 and f2 in the base stations 111, 112 and 113 in normal operation state, all the carrier frequencies f0, f1 and f2 are used for downlink before time t1, and switched to be used for uplink at the same time t1. However, if different switching times between uplink and downlink are used in respective base stations of each cell, interference will occur irrespective of whether smart antenna or group division transmission or group division receiving are adapted in respective base station of each cell, for example, if switching time t1 between uplink and downlink is used for the carrier frequencies f0, f1 and f2 in the base stations 111, while switching time t2 between uplink and downlink is used for the carrier frequencies f0, f1 and f2 in the base stations 112, and switching time t3 between uplink and downlink is used for the carrier frequencies f0, f1 and f2 in the base stations 113, the uplink of base station 111 will be strongly interfered by the downlink of the base station 112 during the time period from t1 to t2 where the downlink and the uplink overlaps, and the uplink of base station 113 will be strongly interfered by the downlink of the base station 112 during the time period from t2 to t3 where the downlink and the uplink overlaps, and the uplink of base station 113 will be strongly interfered by the downlink of the base station 111 during the time period from t3 to t1 where the downlink and the uplink overlaps. Since the antennae of the base station are normally quite high and the transmission power thereof are much higher than user terminals, the base station is not able to receive the signal from the terminal correctly during the time period where the above interference occurs such that normal communication cannot be performed during these time period.
In a TDD base station using multiple carriers, if different switching times between uplink and downlink are adapted for each carrier, and if the above mentioned problem of interference caused by different switching times between uplink and downlink used for multiple carriers in multiple base stations of multiple cells are resolved by radio resource control technology, the flexibility of the system can be increased by means of smart antenna technology.
However, it is not possible for the conventional TDD base station to operate like this, since each set of radio receiver/transmitter supports multiple carriers and the same switches or circulators 221, 222, . . . , 22n are used for each set of radio receiver/transmitter to switch between uplink and downlink and to use the same antenna and feeding cable system.
An obvious technical advantage of TD-SCDMA system which is one of the standards of the third generation mobile communication system is the ability of supporting asymmetric services. However, in conventional base stations using multiple carriers, since the same receiver/transmitter is used for multiple carriers, the switching between uplink and downlink thereof must be performed at the same time, and if the same carrier frequency is used in adjacent cells in the scheduled region, which is a basic requirement of the CDMA system, frame synchronization are usually required and the configuration of the same uplink and downlink ratio is adapted to avoid interference between adjacent cells to the most extent, the flexibility for the system to support asymmetric uplink and downlink service is constrained.